NASA Technical Reports Server (NTRS) 19970026584: Magneto-Hydrodynamic Damping of Convection During Vertical Bridgman-Stockbarger Growth of HgCdTe

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,/C -{( }_ "J& .....
JOURNOAFLC TAL
GROWTH
J,._urnal of Crystal Growth 167(1996) 478-487
Magneto-hydrodynamic damping of convection during vertical
Bridgman-Stockbarger growth of HgCdTe
D.A. Watring *, S.L. Lehoczky
Space Sciences L_d_oratoCv, NASA Marshall Space Flight Center. Huntsville. Alabama 35812, USA
Received 3April 1995; accepted 26 January 1996
ELSEVIER
Journal of Crystal Growth
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......... CRYSTAL
GROWTH
ELSEVIER Journal of Crystal Growth 167(1996) 478-487
Magneto-hydrodynamic damping of convection during vertical
Bridgman-Stockbarger growth of HgCdTe
D.A. Watring *, S.L. Lehoczky
Space S_iem'es LrtboratoO', NASA Marshall Space Flight Center, Huntxcille. Alabama 35812. USA
Received 3April 1995; accepted 26January 1996
Abstract
In order to quantify the effects of convection on segregation, HgosCdoxTe crystals were grown by the vertical
Bridgman-Stockbarger method in the presence of an applied axial magnetic field of 50 kG. The influence of convection, by
magneto-hydrodynamic damping, on mass transfer in the melt and segregation at the solid-liquid interface was investigated
by measuring the axial and radial compositional variations in the grown samples. The reduction of convective mixing in the
melt through the application of the magnetic field is found to decrease radial segregation to the diffusion-limited regime. It
was also found that the suppression of the convective cell near the solid-liquid interface results in an increase in the slope of
the diffusion-controlled solute boundary layer, which can lead to constitutional supercooling.
1. Introduction magnetic fields of 1300 and 1750 G were sufficient
to suppress turbulence in the melt. This eliminated
For nearly four decades, the usefulness of applied the temperature fluctuations which caused melting
magnetic fields in crystal growth from the melt has and resolidification of the growth interface and an
been recognized. The fundamental basis for the inter- associated fluctuation in the dopant concentration in
action between magnetic fields and convection was the crystal grown in zero field.
first discussed in Refs. [1-5]. The basic mechanism Temperature and solutal gradients in the melt
for the interaction of an applied magnetic field and a during growth almost always result in buoyancy-
molten semiconductor involves the electrical currents driven convection. Frequently, this convective flow
induced by the movement of a conductor in the is oscillatory [8] and gives rise to a fluctuating rate
presence of a magnetic field. Early experiments by of crystal growth, which, in turn, produces a micro-
Utech and Flemings [6,7] investigated the effects of a scopically non-uniform distribution of dopant in the
magnetic field on tellurium-doped indium anti- crystal. The universal effect of an applied magnetic
monide grown in a horizontal furnace by directional field is the damping of the convective turbulence in
solidification. Their results indicated that vertical the melt, which produces a more homogenous dopant
distribution [9-13].
The majority of the experimental studies with
magnetic fields has been focused on the growth of
"Corresponding author. GaAs, Si and Ge by the Czochralski growth process.
0022-0248/96/$15.00 Copyright ;t: 1996 Elsevier Science B.V. All rights reserved
I'll S0022-0248196)00279-5

D.A. Watring, S.L Lehoc;kv / Journal O!Crystal Growth 167 (/996) 47S 4S7 47t)
In these systems, small (2-5 kG) fields were applied 2. Experimental procedure
to suppress turbulent convection. Limited work
[6,12-16] has been conducted on the magnetic ef- 2.1. Sample prepa_ttion
fects of crystal growth in a vertical Bridgman-
Stockbarger configuration and out of these experi-
The starting materials were triple-distilled instru-
ments only two involved the II-VI compounds of
ment grade Hg from Bethlehem Apparatus and six
HgCdTe and HgZnTe.
nines grade Cd and Te from Johnson Matlhey. The
The Hg-based II-VI semiconductor compounds
ampoules were made from 8 nnn ID X 12 mm OD
are important for application of infrared detection
commercial grade. T08, fused silica quartz. A ta-
and imaging applications for a broad range of wave-
pered section was lo.-med on the quartz ampoule to
lengths from 0.8 #m to the far-infrared spectrum
enhance the probability of single crystal growth. An
beyond 30 /._m. The crystal growth of these Hg-based
iriternal layer of graphite ,aas formed on the ampoule
II-VI systems is characterized by a destabilizing
as described in Rcf. [Ib] to prevent the adhesion of
horizontal temperature gradient due to a difference in
HgCdTe to the ampoule walls. The elemetlts `,`,crc
the thermal conducti'+'ities of the melt and the crystal
weighed out for Hg I ,Cd,Te (x= 0.2). loaded into
at the growth interface in the presence of a contain-
the ampoules and sealed off under a I('1 -i Torr
ing crucible and the release of latent heat. Addition-
vacuum. The Hg, sCd._,Te ingots were compourlded
ally, a stabilizing vertical solutal gradient is pro-
by a homogenization process that has been described
duced by the rejection of the denser constituent
in detail elsewhere [16]. The formed ingots were 14
(HgTe in the case of HgCdTe) into the melt. These
crn long and wei,.zhed approximatel\' 47 <,
phenomena, coupled with a large solutal-to-thermal
expansion coefficient ratio ( fl<Q/fltAT.. = 100) and
2.2. Directional solidilh'ation growth xvstem
a large thermal-to-solutal diffusion coefficient ratio
(C_T/D k = 200), give rise to double diffusive con-
vection during the growth of these binary semicon- The ingots were regro'+`,'n by' directional solidifica-
ductors, where AT, is the radial temperature differ- tion in the presence of a stationary axially aligned
ence, Co is the starting composition, /3< and /3t are, magnetic field as shown schematicall3 in Fig. la.
respectively, the solutal and thermal expansion coef- The Bridgrnan-Stockbarger crystal growth systcn-i
ficients and o_T and DL are the thermal and solutal consisted of five heated zones with the booster and
diffusion coefficients, respectively. If this cold zone separated by, a 2 cm adiabatic zone. A 0.3
buoyancy-induced convection is large as compared cm thick heat extraction phlte was phiced bel,acen
to the solidification velocity, it can interfere with the adiabatic zone and cold zone to produce high
segregation near the solid-liquid interface resulting axial gradients. The ampoule ,,,+'as suflported by an
in a non-homogenous crystal. It is believed that the inconel 625 cartridge assembly shown in Fig. Ib and
reduction of convection should be advantageous in remained stationary during crystal grm`,'th. Translat-
maintaining the solid-liquid interface shape required ing the furnace instead of the cartridge mininfized
to minimize the crystal defect densities while mini- any movenaenl and vibrations ol+ the sample and
mizing compositional variation trans'+,erse to the allowed the crystallization Io take place in the ho-
crystal growth direction. nlogenous region of the nlagnelic field. The super-
This paper focuses on the influence of the mag- conducting magnet is inanually set to the desired
netic field on mass transfer in the melt and radial field strength (up to 50 kG) and is held constant
segregation at the solid-liquid interface. We begin during the entire growth process. The therinal profile
with a description of the charge crucible configura- was chosen in order to produce a gradient of approx-
tion, followed by a brief discussion of the Bridg- imately 80°C/cm on the ampoule wall at the posi-
man-Stockbarger growth system used for the experi- tion of 706_'C, the solidus ten-_pel'ature for the
ments. Results of the axial and radial compositional steady-state growth [17]. The thernlal profile `,\as
distributions with and without the presence of the translated at a relatively slow rate of 0.2 ttln/s in
magnetic field are, then, described. order to avoid constitutional supercooling [I 8].
480 D.A. Watring, S.L. Lehoczky / Journal of Crystal Growth 167 (1996) 478-487
2.3. Characterization For x values of 0.18 or larger, the radial mi-
crodistribution of cadmium telluride was quantita-
The effect of convection on segregation was de- tively determined from the transmission edge of the
termined by measuring radial and axial composi- IR transmission spectrum. The details of this highly
tional variations in the grown crystals. For this study, automated transmission-edge mapping technique are
the average x values (x is the mole fraction of described in Ref. [22]. Briefly, the hardware consists
CdTe) of wafers cut transverse to the growth direc- of a Fourier transform spectrometer specially
tion were determined by high precision density de- equipped with a software controllable xy stage driven
terminations and the values of the crystal lattice by stepper motors. The stage can position the sample
constant published by Woolley and Ray [19] as throughout a 2.54 cm square in 50/zm steps. Spectra
described in detail elsewhere [20,21]. are then analyzed to obtain the cut-on wavelength
LHe Fill Port
LN 2Fill Port
1 (a)
LN 2Dewar
Bridgman-Stockbarger
Superconducting Growth System
Magnet
1 (b) Alumina lower ampoule support Alumina upper ampoule support
Cartridge / t Threaded interface
Quartz ampoule uartz tube
!
HgCdTe _
Thermocouple type K
Fig. 1. (a) Schematic representation of the magnetic Bridgman-Stockbarger growth system. (b) Containment cartridge utilized for the
magnetically stabilized growth of HgCdTe.
D.A. Warring, S.L. Lehoczky / Journal of Co'stal Grou'th 167 (1996) 478 487 481
and the mole fractions of CdTe are calculated from positional profile never reaches a steady-state value.
the compositional dependence of the energy bandgap. Therefore, improved radial segregation is gained at
For those samples with a cadmium telluride content the expense of less uniform axial composition. If the
of less than x = 0.18, the radial compositional varia- translation rate is increased, as in MCT-L6, shorter
tions were determined by energy dispersive X-ray transients result in steady-state axial compositional
spectroscopy analysis (EDX) using pseudo-binary distribution, but radial segregation increases by two
solid solutions as standards [23]. orders of magnitude. These results are in qualitative
agreement with the numerical calculations of Mo-
takef [24,25]. His results indicate that radial segrega-
3. Results and discussions tion increases initially with decreasing Peclet mass
transfer number reflecting the reduction in the mix-
3.1. Axial compositional distributions ing of the melt, which results in increased non-uni-
formity of melt composition at the growth interface.
Fig. 2 shows the limiting axial compositional This suggests that improved material compositional
distributions for directional solidified HgCdTe. The uniformity requires sufficiently fast growth rates to
two experimental curves [21], illustrate the effects of produce short initial transients and a simultaneous
varying translation rates. The translation rates for reduction of radial segregation caused by convective
ingots MCT-L6 and MCT-L7 were 0.310 and 0.068 effects.
/zm/s, respectively. Solidification at the slower rate Fig. 3 shows the axial compositional distribution
results in the build up of a much longer solute profiles for two crystals: curve MCT-D2 was ob-
boundary layer. Hence, the stabilizing solutal forces tained for growth without a magnetic field and curve
are expected to be less during the growth. This MCT-4 for growth with melt stabilization by a 50
reduced stabilizing force can lead to the increase of kG axial magnetic field. The theoretical curve based
natural convective mixing, which results in a more on a one-dimensional diffusion equation is also
uniform radial segregation. However, the axial corn- shown.
0.6
\
0.5
MCT-LT, R =0.068
A,
_ gm/_
f-_ 0.4 .... A-- MCT-L6, R= 0.310
\\ _ .am/see
C9 N
,, "'\ _ -- Pfann's Equation
.o
0.3
r_
I/
:_ 0.2
Crad =0.0008 y,/
0.1
Crad =0'1500 _ _'
I I I I I I I I I ' -- '
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
Axial Dislance (cm)
Fig. 2. Axial compositional limRs of directional solidification for HgCdTe.
482 I).A. I,_&ltrilt_Z, S.L. Leho('zky / Journal _[' Crystal Growth I(_7 (1996) 478-487
0.6
0.5 ] -- I-D Diffusion (Model)
/t MCT-D2 nofield
* MCT--4, 50kGfield
0.4
\ © IR,MCT-.4, 50kGfield
_ 03
02
Ol
0 2 4 6 8 10 12 14
Axial Distance (em)
Fig. 3. Comparison of the experimental axial compositional distribution to the one-dimensional diffusion model.
no magnetic field applied magnetic field
g,.[-
,'A
5
constitutionally
supercooled
t.z) ", region
(from M. Flemings)
Co
Cd rich areas fonn due to
Furnace temperature, TA, is larger homogeneous nucleation. These
than meh temperature, TM. Stable Cd rich areas, being less dense than
growth occurs. surrounding environment rise to top
of ampoule. Where they remeh and
remain until thc end of growth.
This resuls in aCd rich band at end
of boule.
Fig. 4, Schematic representation of the c,,)n',,rectivc melt flows and solute botmdary layer configurations for directional solidification growth
of Hg{'dTc v,ith and ',_ithout magnetic stabilization.
D.A. Warring. S.L. Lehoc-ky/Jourmd ¢_fCowtal Growth 167 _1996) 47S 487 4_3
The curve for MCT-D2 indicates that, as ex-
pected, during solidification in zero field the distribu-
tion of CdTe undergoes an initial transient, then a
steady-state region followed by a final transient. This
compares well with the curve calculated from an
exact numerical solution to the one-dimensional dif-
fusion equation for an x = 0.2 alloy [26,27]. The rich precipitates Ihnl
have risen Io lop ofboule
curve for MCT-4 (50 kG field) shows a similar due toconsfilutional
initial transient and steady-state section. However, a supercooling
sharp increase in the CdTe content is observed near
the last-to-freeze end just before the final transient.
To ensure that this rise was not due to measurement
error, IR compositional measurements were taken at
various axial locations. There was excellent agree-
ment between the two measurement techniques, see
Fig. 3. This rise in CdTe content may be explained
by constitutional supercooling ahead of the interface.
Fig. 4, shows a schematic of the convective melt
flows and solute boundary layer configurations for Fig. 5. Back-scatter micrograph of t_ quenched HgZnT¢ ingol
the no-field and field cases. In the no-field case, a showing Zn-rich precipitates lll_tl have migrated to Ihe I(_l'_of the
boule due Io Stokes illigratioi1 brought about b} cotlslitttlh+l_tl]
region with intense convection (as compared to the
supercooling [33].
growth rate) caused by radial thermal gradients is
assumed to occur between a bulk of diffusion-con-
trolled melt and the growth interface as proposed by of this phenomenon is demonstrated in Fig. 5. which
Kim and Brown [28] to explain axial segregation illustrates a back-scattering electron micrograph of a
patterns seen in HgCdTe crystal growth experiments quenched HgZnTe ingot grown by Lehoczky et al.
by Szofran and Lehoczky [29]. Hence, the axial [33]. It clearly shows Zn-rich precipitates Ih;alh_tve
temperature gradient in the melt is sufficient to avoid migrated to the top of the boule due to Stokes
constitutional supercooling due to the flatness of the migration resulting from constitutional supercooling.
compositional profile and normal solidification pro-
ceeds. In the case of growth in the presence of a 3.2. Ra_fial coml_ositioJud distribution,s"
stabilizing magnetic field, the elimination or reduc-
tion of convection allows for the build up of the 3.2.1. Radial segregation." initial trallsients
diffusion-controlled solute boundary layer closer to Fig. 6 shows the radial compositional variations
the solid-liquid interface. As the axial temperature for wafers 2.5 cm from the first-to-freeze portion of
gradient is held constant, this stiffer solutal boundary ingots MCT-D2 (zero field) and MCT-4 (50 kG
layer produces a region in the liquid ahead of the field) as determined by IR transmission-edge mea-
interface that is at an actual temperature below its surements. The pattern of low Cd content al the
equilibrium liquidus temperature. Accordingly, this center (high Hg content) and high Cd content along
can lead to homogeneous nucleation of solid-phase the edge of wafer #2.2, prepared from the MCT-D2
particles, richer in Cd (higher x) than the bulk melt, ingot, is consistent with other results obtained prc_i-
which float upwards because their densities are less ously in the absence of a magnetic field. As noted
than those of the melts. This phenomenon of buoyant earlier, the vertical directional solidification of Hg-
rising of solid particles, or Stokes migration, has based II-VI systems is characterized by double dif-
been observed during vertical Bridgman-Stock- fusive convection and a concave (toward the solid)
barger growth experiments of HgCdTe [30,31] and growth interface. Thus, the denser Hg-rich melt tends
HgCdSe [32] and is consistent with the results from to accumulate at the center of the crucible resulting
the growth of the MCT-4 ingot. Conclusive evidence in the radial compositional profile shown in Fig. 6a.
484 D.A. Watring, S.L Lehoczky / Journal of Crystal Growth 167 (1996) 478-487
Fig. 6b shows the radial profile for growth in the Briefly, they solved the species continuity equation
presence of the stabilizing magnetic field. The order for the radial solute concentration in the solid crystal
of magnitude improvement in radial homogeneity at the solid-liquid interface with the assumptions of
clearly indicates that the magnetic field was effica- no convection in the liquid and that the solid-liquid
cious in suppressing convective effects on segrega- interface could be represented by a Fourier series.
tion. Fig. 6c shows the composition variation across Their work showed that in the limiting case, the
the diameter of the two samples. Compositional vari- transverse segregation in the solid is proportional to
ations in other wafers in the initial transient region the deviation of the interface from planarity, the
show similar behavior. proportionality factor being just the product of the
unperturbed concentration gradient and the distribu-
3.2.2. Radial segregation: steady-state region tion coefficient given by
In an effort to quantify the effects of the magnetic
ac/co = (k- I)(R/DL)a_', (3._)
field on the radial segregation, the experimental re-
sults were compared to the analysis of the lateral where AC is the difference in composition at the
solute segregation associated with a curved solid- edge and center of the wafer, k is the equilibrium
liquid interface during steady-state unidirectional so- segregation coefficient, R is the translation rate and
lidification of a binary alloy as derived by Coriell A( is the interface deflection. These calculations are
and Sekerka and described in detail in Ref. [34]. applicable for crystal growth in a stabilizing mag-
0,50
°°°l o.'°
o.,o
_ 0.20
O. 0.(1_
-__
0.45
(¢) _MCT-4 (50 kG)_
0.40
O
ff 0.35
.o
0.30
O
0.25
0.20 ..................... '....... '................... iiiii
-4 -3 -2 -1 0 1 2 3
Radial Position (mm)
Fig. 6. Compositional profiles for (a) MCT-D2 (wafer #2.2) 2.5 cm from first to freeze, grown without a magnetic field, (b) MCT-4 (wafer
#4) 2.5 cm from first to freeze, grown with a 50 kG stabilizing magnetic field and (c) comparision of radial profiles.